Expanded mineral particles are used in a wide range of applications such as insulation, refractories, admixtures for aggregates, soil mixes and the like, where a desired goal of the producer is to transform minerals, such as perlite and vermiculite, from their original mined state into lightweight expanded matter having certain predetermined physical characteristics at a reasonable cost.
The majority of furnaces employed to expand minerals in operation today are energy inefficient. A typical furnace consists of an oil or gas burner and a venturi to accelerate the air/fuel mixture into a tuyere mounted below a vertical tube through which crude mineral ore to be expanded is fed into the open flame. Cold atmospheric air is drawn into the venturi and up into the furnace through a negative draught condition from the exhaust fan. A forced draught fan provides combustion air to the burner. The fuel cloud enters the furnace at the venturi which is positioned directly above the burner head. The combustion air and fuel, ejected at high velocity from the burner head travel through an open gap before reaching the venturi. This condition naturally draws in surrounding cold air into the furnace. For the purposes of this description, this type of combustion setup is referred to as having an atmospheric coupling; i.e. the burner is not fully enclosed. The flame can be directly seen through the open space below the venturi and also through an open port typically fitted onto the side of the tuyere to release to atmosphere under expanded ore and impurities from the continuous process. The fuel air mixture lights and burns through the venturi and tuyere as the flame advances upwards into the vertical expansion chamber. The flame and hot gases heat a steady stream of mineral particles fed into the furnace. In the case of perlite the ore granules glow red as they are heated through the plastic stage. Water of crystallization and other gases that are present in the hot glassy perlite are released as vapour, causing an immediate and significant expansion. Once expanded, the particles, being sufficiently lighter in weight, are carried to the top of the furnace by the process air. Not all the ore fed to the furnace fully expands. Impurities may be present in small amounts which never expand; i.e. obsidian in the case of perlite. These particles must be continuously withdrawn from the combustion zone failing which combustion becomes erratic and the operation shuts down. For this reason alone, systems using atmospheric combustion are preferred. This method always leaves a space where unexpanded mineral can simply fall out of the furnace through a gap and thereby results in a continuous operation. In furnaces designed with atmospheric combustion the pressure above the venturi is near zero so that an opening on the tuyere's slope can be advantageously made to let out undesirable higher density ore, impurities and partly fused agglomerates. This opening also serves to see the flame particle turbulence where it is greatest. The expanded mineral is subsequently separated from the hot process air in cyclones or other separators, cooled, graded and packaged. The exhaust air is diluted with cooling air and filtered in a bag house filter. An induced draught fan pulls the air through the furnace and bag filter and discharges to atmosphere.
In recent years, many attempts have been made to achieve higher efficiency expanded mineral production. The approach often used is to pressurize the furnace by having the burner fully enclosed and having a furnace construction that preheats the combustion air by having it pass outside of the expansion chamber as fully described in U.S. Pat. No. 2,639,132, May 19, 1953 J. H. Bradford. However these systems are much more expensive to build and maintain. The positive operating pressure within the expansion chamber presents a challenge and risk as thermal cracks commonly appear in this chamber which are typically replaced annually. Once cracks form, hot gases under pressure escape. Additionally a separate solution to unexpanded ore dropouts must be provided and direct visual access to the flame is lost as there can be no open port on the side of the tuyere as is the case in the furnace with the typical atmospheric coupling.
There are many installations of furnaces that are employed to expand minerals such as perlite and vermiculite, oxides of metals or otherwise thermally treat mineral particles on a continuous basis.
As an example, U.S. Pat. No. 6,244,860 relates to an apparatus for producing perlite granules, which is having an expansion tube and one or more burners arranged at one end of the expansion tube. The combustion air is blown into the flame through one or more combustion air nozzles. An additional subsonic nozzle is arranged in at least one combustion air nozzle. In this process for expanding raw perlite in the updraft from a flame, with the introduction of oxygen or oxygen-enriched air into this flame, the oxygen or oxygen-enriched air is introduced axially into the flame through subsonic nozzles.
Additionally, in U.S. Pat. No. 2,639,132 (James H. Bradford), there is provided a processing furnace for discrete solids having a fully enclosed and therefore pressurized burner. The processing furnace provides for a closed waste compartment at the base of the furnace fitted with a door for waste removal at convenient intervals. Such a door, if opened during operation, would issue hot process gases under pressure which is dangerous and not recommended. Therefore the furnace needs to be shut down periodically for clean out thereby loosing the efficiency benefits of a continuous operation.
Moreover, U.S. Pat. No. 4,347,155 (Kenneth L. Jenkins) shows preheated combustion air supplied to a fully enclosed burner and is subject to the positive pressure as developed by the combustion air fan and additionally shows a secondary air pipe for admitting preheated air above the venturi into a second enclosure which surrounds at least a portion of the wall of the expansion chamber. It is to be noted that this device is fully enclosed, operates under pressure and does not provide this secondary pre-heated air which is admitted below the venturi around the burner head nor does it provide for a tertiary stage pre-heated combustion air stream around the venturi.
Current practice is for the combustion to take place at the base of the expansion chamber which is open to atmosphere. In the case of perlite, the expansion operation typically requires four to five times (400-500%) more energy than the theoretical heat calculations suggest. Most of this heat is contained within the exhaust air which is rejected to atmosphere and seldom used.
Because the furnace is open to atmosphere at its base, cold air is naturally drawn into the furnace which dilutes the hot flame and this adversely affects overall efficiency. This cold air drawn in at the base is referred to as ‘dilution’ or ‘secondary air’. Some additional air above the stoichiometric ratio is useful to ensure complete combustion and to assist in lifting the expanded perlite out of the furnace however it has been observed that 40-60 percent excess air is typically drawn in at this point which adversely affects the efficiency of the expansion operation.
For these disadvantages established, there is therefore a need for an improved furnace device to offset the quenching effect of the cold air normally drawn in at that point and to increase conversion and yield in any process of thermal expansion of mineral particles.